1. Field of the Invention
This invention relates to surface controlled subsurface safety valves for controlling fluid flow within a well. The invention also relates to a pilot valve for communicating control fluid to the safety valve from the well surface.
2. Prior Art
A common limitation of conventional surface controlled subsurface safety valves is the volume of control fluid which must be displaced from the valve's control chamber to permit valve closure. Preferably, safety valves are designed to close when control fluid pressure drops below a preselected value. Thus, damage to the wellhead and/or control system should result in closure of the safety valve at its downhole location. For conventional safety valves operated by a single control fluid conduit from the well surface, various fluid forces resist valve closures. These forces include the hydrostatic pressure of fluid in the conduit, inertia of fluid in both the conduit and the control chamber of the safety valve, and friction forces developed between the control fluid and the interior of the conduit.
U.S. Pat. No. 4,119,146 to Frank H. Taylor and U.S. Pat. No. 4,173,256 to Marion D. Kilgore disclose pilot operated subsurface safety valves with improved response time for closure of the respective safety valves. Both patents are incorporated by reference for all purposes within this application.